1. Field of the Invention
The present invention relates to a servo controller for driving a load machine, such as a feed shaft for use with a machining tool or an arm for use with an industrial robot, by using a motor. More particularly, it relates to a servo controller for performing path control on a machine having two or more axes.
2. Description of Related Art
There have been provided prior art servo controllers for carrying out feed forward control so as to compensate for a delay in response to an instructed value of an amount to be controlled such as the position or speed of a target machine. For example, Japanese patent publication No. 2762364 (reference 1) discloses a servo controller for differentiating a position instruction signal so as to obtain an amount of feed forward control associated with the position of a target machine, for adding the amount of feed forward control to an amount of control acquired by carrying out position loop control so as to obtain a speed instruction signal, for adding an amount of feed forward control associated with the speed of the target machine, which is obtained by differentiating the amount of feed forward control associated with the position of the target machine, to a value acquired by carrying out speed loop control so as to obtain an electric current instruction signal, and for performing servo control, thereby improving the response of position control (see FIG. 1 of Japanese patent publication No. 2762364, for example).
Japanese patent application publication (TOKKAI) No. 2000-92882 (reference 2) discloses a servo controller in which a simulated control circuit is so constructed as to control a mechanical system model which is approximated as a two-inertia oscillation system and is provided with a torque transmission mechanism, a load machine, and an electric motor, and to add the position, speed, and torque of a simulated electric motor of the simulated control circuit to a value acquired, as an amount of feed forward control, by carrying out position loop control and speed loop control, thereby improving the response of position control without exciting vibrations even when the stiffness of the target to be controlled is low and the target to be controlled has resonance characteristics (see FIG. 25 of Japanese patent application publication (TOKKAI) No. 2000-92882, for example).
A problem with the prior art servo controller disclosed in Japanese patent publication No. 2762364 is that while the prior art servo controller can offer adequate performance when the stiffness of the target to be controlled is high and the target to be controlled can be assumed to be a rigid body, mechanical resonance vibrations can cause vibrations in the position and speed of the target to be controlled, which are amounts to be controlled, when the prior art servo controller is applied to a target to be controlled having low stiffness and resonance characteristics and the response of position control is increased, the positioning accuracy and path tracking accuracy decrease, as shown in FIG. 13.
A problem with the other prior art servo controller disclosed in Japanese patent application publication (TOKKAI) No. 2000-92882 is that when the target to be controlled can be assumed to be a two-inertia oscillation system, while the position of the target to be controlled completely responds to the position of the simulated control circuit at all times and therefore the response of the position control can be improved without exciting vibrations, the simulated control circuit constitutes a feedback control system and therefore the impulse response is not made to become symmetric. As a result, the response path of the target to be controlled does not become symmetric even if a symmetric path is provided as the instructed path, and therefore, when the target to be controlled is instructed to travel between two positions along the same instructed path so that the direction of travel is changed as shown in FIG. 14, a difference can occur between the two response paths of the round trip. This results in the generation of scratches on a machined surface of a mold when the mold is machined with reciprocating machining.